allow list of excluded or included parameters for saving

[bob-carpenter]

Summary:

Match the RStan feature that lets you indicate which parameters to save by name (not by index within name).

I'm not sure if this is something that's best to filter down at the Stan level. @syclik?

Current Version:

v2.15.0

[MatsuuraKentaro]

Hi, is there any progress? Without this, I cannot comfortably use transformed parameters and generated quantities block in a little complicated model... (I need to write all the big variables in model and generated quantities block twice.)